Publications

2017

Résumé: Marine intrusion was simulated in a choked and in a restricted coastal lagoon by using a 3D-hydrodynamic model. To study the spatiotemporal progression of seawater intrusion and its mixing efficiency with lagoon waters we define Marine Mixed Volume (V-MM) as a new hydrodynamic indicator. Spatial patterns in both lagoons were described by studying the time series and maps of VMM taking into account the meteorological conditions encountered during a water year. The patterns comprised well-mixed zones (WMZ) and physical barrier zones (PBZ) that act as hydrodynamic boundaries. The choked Bages-Sigean lagoon comprises four sub-basins: a PBZ at the inlet, and two WMZ's separated by another PBZ corresponding to a constriction zone. The volumes of the PBZ were 2.1 and 5.4 millions m(3) with characteristic mixing timescale of 68 and 84 days, respectively. The WMZ were 123 and 433 millions m(3) with characteristics mixing timescale of 70 and 39 days, respectively. (C) 2017 Elsevier Ltd. All rights reserved.

Résumé: ABSTRACT: In the context of increasing demand for environmental recovery, aquatic systems may face the challenge of evolving under oligotrophication. This is the case in Mediterranean lagoons, in particular the shellfish-farmed Thau lagoon in France, where we studied recruitment of the Pacific oyster Crassostrea gigas. Oyster spat and environmental parameters were monitored at several sampling sites for 3 yr (2012 to 2014) using an original method with a temporal overlap deployment of collectors to study pre- and post-settlement processes and to identify the best conditions for recruitment. Contrary to the ‘no Pacific oyster reproduction’ paradigm in Mediterranean lagoons, our study showed that recruitment of this introduced species is possible in the Thau lagoon at levels comparable to those in other traditional French breeding basins. We identified a favorable environmental window for recruitment characterized by high water temperature (>26.5°C) and high nanophytoplankton and Chaetoceros spp. abundances (>4.3 × 106 and 345 × 103 cells l-1, respectively). In these favorable conditions, we hypothesize that the ecosystem functions as an autotrophic system, in contrast to the heterotrophic system that characterizes unfavorable conditions. Under heterotrophic conditions, high abundances of mixotrophic and heterotrophic organisms (ciliates and dinoflagellates) limited the metamorphosis of C. gigas larvae, leading to poor recruitment. This study provides new knowledge on the reproduction of the Pacific oyster in a Mediterranean lagoon under warming and oligotrophication. The shellfish industry will profit from the discovery of spatfields to develop new nursery practices that are eco-friendly and limit risks of transfers with other spatfall areas.

Résumé: The Thau lagoon, located at the South of France, provides 10% of the French pacific oyster Crassostrea gigas production. Despite this intensive shellfish production, the larval recruitment of this species within the Thau lagoon is still misunderstood and spat collection remains highly variable; as a consequence, shellfish farmers rely mostly on hatchery”s spat. The present study aimed to describe reproduction features of C. gigas within the Mediterranean, as well as to explore spawning triggers of this species. To achieve this goal, the reproduction cycle from gametogenesis to spawning was followed during two years in several locations within the Mediterranean Thau lagoon. The spawning behavior of C. gigas within the Thau lagoon appeared to be slightly different from the one of the Atlantic Coast, showing several spawning events from June to October. The minimal temperature for massive spawning was 23 °C, which is much higher than temperatures mentioned in the literature for this species. A strong relationship was found between phytoplankton communities and gametogenesis, the latter being improved by a higher diatom/dinoflagellate ratio. Temperature was insufficient alone to explain spawning, highlighting other potential triggers such as the moon or lightning strikes. We made the hypothesis of a hierarchy of events triggering spawning, encompassing locking, synchrony and intensifying triggers.
Statement of relevance
This paper is filling in a gap in the literature regarding to reproduction of C. gigas in the Thau lagoon, which is always associated to temperature. However, temperature may not be the only factor involved in oyster reproduction, that is what we tried to show in this paper.

2014

Résumé: Although spatial studies of diseases on land have a long history, far fewer have been made on aquatic diseases. Here, we present the first large-scale, high-resolution spatial and temporal representation of a mass mortality phenomenon cause by the Ostreid herpesvirus (OsHV-1) that has affected oysters (Crassostrea gigas) every year since 2008, in relation to their energetic reserves and the quality of their food. Disease mortality was investigated in healthy oysters deployed at 106 locations in the Thau Mediterranean lagoon before the start of the epizootic in spring 2011. We found that disease mortality of oysters showed strong spatial dependence clearly reflecting the epizootic process of local transmission. Disease initiated inside oyster farms spread rapidly beyond these areas. Local differences in energetic condition of oysters, partly driven by variation in food quality, played a significant role in the spatial and temporal dynamics of disease mortality. In particular, the relative contribution of diatoms to the diet of oysters was positively correlated with their energetic reserves, which in turn decreased the risk of disease mortality.

Résumé: We present the first large-scale, high-resolution spatial and temporal pattern of disease mortality caused by Ostreid herpesvirus 1 (OsHV-1) that has affected Pacific oyster Crassostrea gigas in France every year since 2008, in relation to local farm use. Mortality was monitored in healthy oyster spat deployed at 106 locations in the Thau Mediterranean lagoon during the epizootic that occurred in spring 2011. We conducted a field survey of farming practices in the areas where the sentinel oysters were placed before the epizootic. We found that the risk of disease mortality in sentinel oysters was higher within the bivalve farming area than outside it, suggesting that the infection pressure is much higher in areas with intensive farming activity. The risk of mortality was higher in farms rearing spat, a developmental stage particularly susceptible to OsHV-1, than in farms rearing adult oysters, which are generally more resistant. Additionally, the mortality risk in farms rearing adult animals was similar to that in empty farms, which suggests that, during the study period, adult oysters did not vector the disease. Interestingly, the mortality risk for sentinel oysters deployed in mussel Mytilus galloprovincialis farms was lower than for those held in oyster farms or empty farms, suggesting that mussels reduced the infection pressure on susceptible oysters.